The invention relates to a flexible optical graded-index profile fiber.
There is a multiplicity of lasers for the visible, near infrared and near ultraviolet spectral regions. E.g., the Nd:YAG lasers radiate at the wavelength of 1.064 microns. Their initial output can exceed 100 W in the continuous-wave operation, and the peak output can be many MW in pulsed operation. Many of these lasers radiate in modes with low order, and so-called Gaussian modes or similar modes often occur.
Generalized mode order m is used for ordering the modes. For Gauss-Hermite modes (typically for lasers with "cartesian geometry"), e.g., m=n.sub.x +n.sub.y +1, and n.sub.x and n.sub.y are the mode order relative to the x and y axis: n.sub.x or n.sub.y is the number of nodes in the intensity distribution perpendicular to the beam axis in x or y direction. For Gauss-Laguerre modes (in cylinder symmetry), m=2.multidot.p+l+1, and p and l are the radial and azimuthal mode order. This classification is known to one skilled in the art, who also uses it correspondingly even if Gaussian modes are not involved in the modes.
In lasers with high output, m is almost always greater than 1. Then, it can hardly be achieved that only one mode occurs--the lasers alternate modes during operation or radiate at the same time in several modes. But there is an upper limit o for generalized mode order m of the radiation of a laser, perhaps since higher modes are not made because of its resonator configuration. Number o is designated as the maximum mode order of the radiation. The overall number of modes which the laser can radiate scales about like.about.o.sup.2.
For the uses, e.g. in material processing, a low mode order is mostly advantageous, i.a., since the radiation can be brought to focus on small focal spots or optical systems for imaging, the radiation can be configured more simply. The mode order therefore can often be connected with the concept of the beam quality: the quality of the laser radiation is the higher the smaller the mode order, and it is distributed over fewer modes. For many uses of lasers with high output, e.g. the beam quality is sufficient if maximum mode order o is less than about 10.
For transmission of the laser radiation with high output, flexible fibers are often necessary. These fibers have to be able to transmit not only high radiation outputs, but they are not to permit the mode structure of the radiation to worsen; thus, e.g., increasing the maximum mode order specified by the laser as little as possible. Fibers for transmission of laser radiation are known in the art, but the known fibers with high output or good mode structure show considerable defects in the transmission of radiation.
For transmission of a basic mode (o=1), monomode fibers are suitable. These can be made as graded-index profile fibers, e.g. as power-law index profile fibers with the refractive index profile EQU n.sup.2.sub.k (r)=n.sup.2.sub.ki -(n.sup.2.sub.ki -n.sup.2.sub.ka).multidot.(2.multidot.r/D.sub.k).sup.g for r less than D.sub.k /2 (1)
with exponent g (n.sub.ki : refractive index in the center of the fiber; n.sub.ka : refractive index on the interface at the fiber cladding, n.sub.ki greater than n.sub.ka ; r: distance from the fiber axis; D.sub.k diameter of the fiber core); but in practice, step index fibers are preferred. However, monomode fibers can transmit no modes with higher order and because of their small core diameter, generally only a few microns, they also can transmit no high radiation outputs. Fibers for transmission of radiation with high output in modes with high order have to be multimode fibers. Their core diameter is at least some tens of microns.
Flexible multimode step index fibers for transmission of laser radiation with high output are known (Schott Product Information 1024e, "Special fibers made of quartz glass," 1989, Schott Glaswerke, Mainz). Multimode step index fibers are unsuitable, however, for transmission of radiation with good mode structure, since their fiber modes (the conditions of radiation in the fiber are designated as fiber modes) are greatly different from the laser modes. In the launching of the radiation, a multiplicity of fiber modes is generally excited.
It is assumed that r.sup.2 fibers can transmit radiation in modes with higher order, especially Gaussian modes (J. A. Arnaud, "Beam and Fiber Optics," Academic Press, New York, 1975, ISBN 0-12-063250-0). In the core of cylindrical r.sup.2 fibers the position dependence of the refractive index follows the relation EQU n.sup.2.sub.k (r)=n.sup.2.sub.ki -(n.sup.2.sub.ki -n.sup.2.sub.ka).multidot.(2.multidot.r/D.sub.k).sup.g =n.sup.2.sub.ki .multidot.(1-(f.multidot.r).sup.g) with g=2 (2)
(r: distance from the fiber axis; D.sub.k : core diameter; n.sub.ki :, n.sub.ka : refractive index in the center of the fiber and on the interface at the cladding, n.sub.ka is less than n.sub.ki ; f: specific convergence, a measurement for the curvature of the refractive index profile). PA1 a) core diameter D.sub.k is between 200.multidot.10.sup.-6 m and 800.multidot.10.sup.-6 m, PA1 b) but D.sub.k is at least 2.5 times (o'.multidot..lambda.)/.sqroot..DELTA.n.sub.k and o' has the value 5, if the maximum mode order of laser radiation o is less than 5, o'=o, if o is in the range of 5 to 20, or o'=4.5.multidot..sqroot.o if o is greater than 20, .sqroot..DELTA.n.sub.k is the root of refractive index difference .DELTA.n.sub.k in the core and .lambda. is the wavelength of the laser radiation to be transmitted, PA1 c) exponent g is between 1.4 and 3.0 PA1 d) and the refractive index difference in the core, .DELTA.n.sub.k =n.sub.ki -n.sub.ka, is greater than 1.6.multidot.10.sup.-3.
Although the operating expense for the achievement of certain refractive index profiles in fibers is very high (K. W. Raine, J. G. N. Baines, D. E. Putland, "Refractive Index Profiling-State of the Art," J. Lightw. Techn. 7, 1162-1169, 1989), a sufficiently good r.sup.2 graded-index profile so far has not yet been achieved.
Multimode fibers with "r.sup.2 -similar" graded-index profiles are known. U.S. Pat. No. 38-23-997 describes, e.g., a fiber for communication engineering uses, whose refractive index profile is optimized with respect to the reduction of the modal operating time scattering (operating time a function of the fiber modes of the mode order). For the exponents of the refractive index profile of this fiber, g approximately equal to 2-(n.sub.ki -n.sub.ka)/n.sub.ki : applies, g has virtually the value 2. DE-PS 27-45-715 describes a fiber with g approximately equal to 1.92 for the same use, for which materials are proposed: the core of the fiber consists of GeO.sub.2 -- and P.sub.2 O.sub.5 -doped SiO.sub.2. Such "communication fibers" are unsuitable for transmission of high radiation output, since the radius of the fiber modes is too small. Further, these fibers worsen the mode structure of the radiation by the transmission, since the fiber modes correspond only slightly to the laser modes. And even though only a few fiber modes are excited in the launching of the radiation in these fibers, the radiation is unselectively distributed on the fiber end to many fiber modes because of the strong coupling of the radiation output between the fiber modes.
To confine the radiation securely in the fiber core and to keep the modal operating time scattering small, the refractive index difference in the core is usually selected large in the known fibers. In the graded-index profile fibers of glass with a SiO.sub.2 base for transmission of radiation in the near ultraviolet to near infrared spectral region, .DELTA.n.sub.k is often greater than 20.multidot.10.sup.-3 or even greater than 30.multidot.10.sup.-1. Large values for .DELTA.n.sub.k require compositions or production processes for the fibers, which do not allow a precise adjustment of the refractive index profile or lead to other drawbacks.
DE-PS 32-61-536 describes, e.g., a fiber, in whose core a refractive index profile is caused by a radially variable doping with metal oxides (MgO, SrO, BaO, etc.). But this fiber is sufficiently conductive only in the spectral region from 1100 nm to 1500 nm.
For fibers for transmission of radiation of the near ultraviolet to near infrared spectral region, GeO.sub.2, P.sub.2 O.sub.5, B.sub.2 O.sub.3 or F dopings are preferably introduced in SiO.sub.2 glass tubes using inner coating CVD processes. GeO.sub.2 or P.sub.2 O.sub.5 dopings and mixtures of these dopants act in Si02 glass in a refractive index increasing manner, B.sub.2 O.sub.3 or F dopings and their mixtures act in a refractive index decreasing manner. A position-dependent refractive index in the tube follows from a position-dependent doping, a glass rod results by collapsing the tube, the fiber results by drawing of this so-called fiber preform.
The B.sub.2 O.sub.3 doping can cause absorption bands or the weakening mechanical strength of the fibers. The F doping is therefore preferred to lower the refractive index. But since the refractive index lowering by fluorine in SiO.sub.2 glass is small and the maximum F portion has to remain small, fibers, in which the refractive index profile is adjusted only by an F doping of the core, are not used.
Usually, the refractive index profile in the fiber is produced by a refractive index increase and for this purpose, the core is doped position-dependently with GeO.sub.2 or P.sub.2 O.sub.5. The GeO.sub.2 doping is especially preferred, since its portion in SiO.sub.2 glass can be high and it greatly raises the refractive index. Fibers whose cladding additionally is doped with fluorine to achieve a greater refractive index difference between the core and the cladding are also known. EP-PA 01-25-828 describes a fiber with a core doped strongly with GeO.sub.2, whose cladding is doped strongly with fluorine in a thin layer around the core, and the F portion in this layer is constant or radially decreases from inside to outside.
Although the influence of the dopants on the refractive index works in a contrary sense, it occurs in isolated cases that the core is doped both with GeO.sub.2 or P.sub.2 O.sub.5 and with fluorine; but the reasons for it are not to be found in the refractive index profile. In the fiber described in EP-PS 01-60-244, e.g. an additional fluorine doping of the core doped mainly with GeO.sub.2 or P.sub.2 O.sub.5 brings about a lower fundamental absorption in the UV region. EP-OS 01-91-202 describes a fiber, whose core contains specific dopants, i.a., also fluorine, to reduce the socalled drawing contribution to the Rayleigh scattering of the fiber--but these "more unpure" fibers have a smaller destruction threshold and are therefore unsuitable for transmission of high radiation outputs.
In inner coating processes with GeO.sub.2 or P.sub.2 O.sub.5, a "refractive index dip," a decrease of the refractive index in the center of the fiber, regularly occurs. The dip results during the collapsing of the glass tubes by evaporation of the doping materials, since its vapor pressure is higher than the vapor pressure of the SiO.sub.2. The dip therefore leads to a strong coupling between the fiber modes and thus to a reduction of the quality of the beam. By careful collapsing, the dip can be reduced. It is known, e.g., from DE-PS 34-19-835 to expose the inside of the tubes to a vapor with a defined composition during collapsing so that the dip is small. Also, it is often attempted to suppress the dip in that the material evaporating during collapsing is already taken into consideration in the doping. However, altogether these processes work only in a slightly reproducible manner.
The refractive index dip could be avoided if the fiber preforms were produced with outer coating or sintering processes, as described, e.g., in EP-PS 01-75-067. These processes, however, have the drawback that the refractive index profile cannot be set finely enough or that the fiber material is not pure enough to transmit laser radiation with high output with the fibers.
DE-PS 28-37-338 describes a multimode fiber for communication engineering uses, in which the influence of the dip is compensated by a suitable configuration of the refractive index profile outside the dip. But this fiber is also not suitable for transmission of laser radiation with high output and good mode structure.